"The Social Security trust funds contain nothing more than IOUs"
While that's technically true, the "trust fund" is already counted in our national debt. We could borrow money to pay SS recipients without affecting the total debt.
For example, if we borrowed $65 billion from China to pay SS this month, that would add $65 billion to China's debt holdings, but subtract $65 billion from the SS trust fund. The sum of the two doesn't change.
Put another way, it's like taking out a home equity loan to pay your mortgage. Your total debt wouldn't change, but your mortgage would get paid. If SS recipients don't get their checks, it has nothing to do with the debt ceiling.
When the "trust fund" runs out, that will be a different story